One of the many hopeful young artists who have flocked to the quaint Greenwich Village area of New York where Dutch attics and low rents cohabit, Sue from Maine rooms with Joanna, known as Johnsy. Because Johnsy is from California, she is not acclimated to the cold; consequently, she contracts pneumonia. Her devoted friend Sue attends to her, but Johnsy worsens.
Having made a house call, the doctor tells Sue, " Your little lady has made up her mind that she's not going to get well." He asks Sue if Johnsy has anyone on her mind, who is worth living for. When Sue replies, "No," the physician explains "it is the weakness, then," meaning the weakness of her mind which keeps her ill. He suggests that Sue get Johnsy interested again in life. But, Johnsy resists Sue's encouragements. In desperation, Sue tells the tenant who lives downstairs that Johnsy is gravely ill and has begun to count the leaves upon a vine outside the window by her bed. Mr. Behrman, the tenant, is enraged that pneumonia could overtake Johnsy and that she has the "foolishness to die because leafs dey drop off from a confounded vine?" Then, he asks Sue how she could allow such silly business to get into Johnsy's mind.
Yet, Behrman acquiesces to Sue's wish that he pose for her. When they look outside at the ivy vine, "Then they looked at each other for a moment without speaking." The next morning Johnsy cannot look out the window because Sue has pulled down the shade. "Put it up; I want to see," she whispers to Sue her order. Sue reluctantly pulls it up. "There yet stood out against the brick wall one ivy leaf."
Johnsy says that she thought this lone leaf had fallen, but it will. Still it remains on the vine even the next day. Staring at that leaf, Johnsy is inspired. She tells Sue that she has been wicked to want to die. She is going to live, and asks for some chicken broth. She tells Sue she will watch her, and someday when she is well she will paint the Bay of Naples.
Johnsy is a character developed through the indirect methods of revealing her thoughts, and her speech and her interactions with others. When old Mr. Behrman climbs from outside and paints the leaf upon the window, Johnsy feels that if that leaf can hold on through storms, she, too, can weather the pneumonia. The power of Johnsy's mind saves her, just as the doctor has told Sue.